Write documentation for ocamldoc comment header syntax

[ryyppy]

In Reason, doc headers need to be written in ocamldoc syntax.

You can find the specification of the ocamldoc format in the OCaml Manual here

Ideally, we'd extract the most relevant information and describe the syntax in Reason snippets, so it's easy to copy and paste them. Maybe give some pointers on best practises ("Comments are usually put into .rei files, if existent").

Infrastructure related information (odoc, ocamldoc, building etc.) can be left out for now.

Useful resources:

• https://ocamlverse.github.io/content/documentation_guidelines.html